Tungsten cathode material

ABSTRACT

A tungsten cathode material to be used for TIG welding, plasma spraying, plasma cutting, electro-discharge machining, discharge lamps, and the like is improved; use of the radioactive element thorium is reduced; and a long life and a high performance are realized. In a tungsten cathode material, oxide particles containing an oxide or oxides of at least one selected from the group consisting of Sm, Nd, Gd, and La in a total amount of 50 vol % or more are dispersed, the oxide particles having an average particle diameter d satisfying the relationship 0&lt;d≦2.5 μm. Given a volume fraction f (vol %) of the total amount of the oxide particles in the tungsten cathode material, 0.083≦f/I holds when a current I(A) such that 0&lt;I≦40 A is applied to an electrode being made of the tungsten cathode material.

TECHNICAL FIELD

The present invention mainly relates to a tungsten material to be usedas a cathode material for use with TIG (tungsten inert gas) welding,plasma spraying, plasma cutting, electro-discharge machining, adischarge lamp, and the like, as well as a discharging device utilizingthe same. It also relates to a discharging method which employs atungsten material for an electrode.

BACKGROUND ART

Tungsten has the highest melting point among metals, and has arelatively good electrical conductivity; therefore, it is widely used asan electrode material for which thermal resistance is required. However,when tungsten is used as a cathode material, a substance having a lowwork function is often added for improved thermionic emissioncharacteristics.

As is shown in Table 1 and Table 2 below, among other additives, ThO₂(thorium oxide) has a high melting point and boiling point, and arelatively low work function. Therefore, W—ThO₂ alloys have been used asexcellent cathode materials. However, since Th is a radioactive element,there are voices against its use, which have led to the aspirations formaterials having superior characteristics to those of conventionalW—ThO₂ alloys.

For example, Patent Document 1 discloses an electrode for dischargelamps, in which a tungsten alloy having an oxide of Pr, Nd, Sm, or Gdadded thereto is used. However, the data disclosed in Patent Document 1illustrates test results for as short a period as 100 minutes, andalloys exhibiting stable characteristics for a long time in practice arenot obtained. Patent Document 2 discloses tungsten sintered bodies towhich oxides of Ce, Th, La, Y, Sr, Ca, Zr, and Hf are added. However, nodetailed data showing the characteristics of the resultant sinteredbodies are described.

TABLE 1 oxide oxide oxide work oxide work melting boiling functionfunction point point (at 0K) (at 1700K) oxide ° C. ° C. eV eV ThO₂ 30504400 2.55 3.07 CeO₂ 2600 3227 3.2 3.21 Lu₂O₃ 2490 — 2.3 3.26 Nd₂O₃ 2272— 2.3 3.3 Sm₂O₃ 2325 3527 2.8 3.21 Tm₂O₃ 2400 — 3.27 — Y₂O₃ 2415 43002.0 3.5 La₂O₃ 2250 4200 2.8 3.1 Pr₆O₁₁ 2200 — 2.8 3.48 Gd₂O₃ 2340 — 2.13.29 Dy₂O₃ 2340 — 2.2 3.18 WO₃ 1473 1837 — —

TABLE 2 metal work metal metal melting function boiling point point (at0K) metal ° C. ° C. eV Th 5484 1785 3.3 Ce 3257 800 2.6 Lu 3315 16613.14 Nd 3127 1025 3.3 Sm 1752 1072 3.2 Tm 1727 1545 3.12 Y 3337 14102.954 La 3454 920 3.3 Pr 3212 935 2.7 Gd 3233 1315 3.07 Dy 2335 14093.09 W 5800 3387 4.52

CITATION LIST Patent Literature

-   [Patent Document 1] Japanese Laid-Open Patent Publication No.    5-54854-   [Patent Document 2] International Publication No. 2005/073418

SUMMARY OF INVENTION Technical Problem

As is also described in Patent Document 1, in the case of tungstenalloys and conventional W—ThO₂ alloys to which these oxides are added,the oxide will diffuse from the interior of the alloy over to thecathode surface. However, although an effect of lowering the workfunction is obtained, it is considered that the diffusion processbottlenecks improvements in characteristics. Moreover, W—ThO₂ alloys arealso employed as electrodes in usages other than discharge lamps;however, it is not made clear as to which material is suitable for thedischarge phenomenon at large.

Thus, in various devices utilizing the discharge phenomenon, e.g.,discharge lamps and electrodes for TIG welding, an electrode materialwhich can replace the W—ThO₂ alloys has been desired.

The present invention has been made in view of the above problems, andan objective thereof is to provide a tungsten cathode material havingexcellent characteristics and a discharging device in which the same isused. Alternatively, an objective of the present invention is to providea discharging method utilizing a tungsten electrode.

Solution to Problem

A tungsten cathode material according to the present invention is atungsten cathode material in which oxide particles are dispersed, theoxide particles containing an oxide or oxides of at least one selectedfrom the group consisting of Sm, Nd, Gd, and La in a total amount of 50vol % or more, wherein, an average particle diameter d of the oxideparticles satisfies the relationship 0<d≦2.5 μm; and, given a volumefraction f (vol %) of the total amount of the oxide particles in thetungsten cathode material, 0.083≦f/I holds when a current I(A) such that0<I≦40 A is applied to an electrode being made of the tungsten cathodematerial.

Alternatively, a discharging method according to the present inventionis a discharging method comprising: a step of providing a tungstenelectrode in which oxide particles are dispersed, the oxide particlescontaining an oxide or oxides of at least one selected from the groupconsisting of Sm, Nd, Gd, and La in a total amount of 50 vol % or more,the oxide particles having an average particle diameter d in the rangeof 0<d≦2.5 μm; and a step of allowing a current I such that 0<I≦40 A toflow in the tungsten electrode, wherein, given a volume fraction f (vol%) of the total amount of the oxide particles in the tungsten electrode,the relationship 0.083≦f/I is satisfied in the step of allowing acurrent to flow in the tungsten electrode.

Alternatively, a discharging device according to the present inventionis a discharging device comprising a cathode and a power supply forallowing a current to flow in the cathode, wherein, the cathode is atungsten cathode in which oxide particles are dispersed, the oxideparticles containing an oxide or oxides of at least one selected fromthe group consisting of Sm, Nd, Gd, and La in a total amount of 50 vol %or more, the dispersed particles having an average particle diameter dsatisfying the relationship 0<d≦2.5 μm; the power supply is arranged toallow a current of 0<I≦40 A to flow in the cathode; and given a volumefraction f (vol %) of the total amount of the dispersed particles in thetungsten cathode material, 0.083≦f/I holds when the current I(A) appliedto the cathode is such that 0<I≦40 A.

Alternatively, a tungsten cathode material according to the presentinvention is a tungsten cathode material in which oxide particles aredispersed, the oxide particles containing an oxide or oxides of at leastone selected from the group consisting of Sm, Nd, Gd, and La in a totalamount of 50 vol % or more, wherein, given an average particle diameterd (μm) of the oxide particles and a volume fraction f (vol %) of thetotal amount of the oxide particles in the tungsten cathode material,1.67≦f/d≦4 holds when a current I such that 10 A≦I is applied to anelectrode being made of the tungsten cathode material.

Alternatively, a discharging method according to the present inventionis a discharging method comprising: a step of providing a tungstenelectrode in which oxide particles are dispersed, the oxide particlescontaining an oxide or oxides of at least one selected from the groupconsisting of Sm, Nd, Gd, and La in a total amount of 50 vol % or more;and a step of allowing a current of 10 A or more to flow in the tungstenelectrode, wherein, given a volume fraction f (vol %) of the totalamount of the oxide particles in the tungsten electrode, an averageparticle diameter d (μm) of the oxide particles and the volume fractionf satisfy the relationship 1.67≦f/d≦4.

Alternatively, a discharging device according to the present inventionis a discharging device comprising a cathode and a power supply forallowing a current to flow in the cathode, wherein, the cathode is atungsten cathode in which oxide particles are dispersed, the oxideparticles containing an oxide or oxides of at least one selected fromthe group consisting of Sm, Nd, Gd, and La in a total amount of 50 vol %or more; the power supply is arranged to allow a current of 10 A or moreto flow in the cathode; and given a volume fraction f (vol %) of thetotal amount of the oxide particles in the tungsten electrode, anaverage particle diameter d (μm) of the oxide particles and the volumefraction f satisfy the relationship 1.67≦f/d≦4.

Advantageous Effects of Invention

According to the present invention, as a tungsten alloy to be used for acathode of a discharge lamp, a TIG welding rod, or the like, anelectrode material having good characteristics with a similar orsuperior long life is obtained without using thorium.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 A schematic diagram for describing a method of measuring a sizeand an added amount of particles.

DESCRIPTION OF EMBODIMENTS

First, the inventors' view of electrodes which are made of tungstenalloys containing rare earth oxides as additives will be discussed. Inthe present specification, a tungsten material or tungsten alloy whichis used as an electrode (cathode) may be referred to as a tungstenelectrode (cathode). A tungsten electrode (cathode) may containadditives such as rare earth oxides. Moreover, a tungsten cathodematerial refers to a tungsten alloy to be used for a tungsten cathode.

In tungsten cathode materials, a rare earth oxide such as Nd₂O₃ to beused as an emitter has a lower boiling point than that of ThO₂, andtherefore evaporates soon off the electrode surface. Therefore, inconventional tungsten cathode materials to which rare earth oxides areadded, diffusion (supply) of the rare earth oxide to the electrodesurface does not occur in time, thus presumably failing to provide along life.

Therefore, by making oxide particles into finer particulate form, it isensured that a large number of interfaces between tungsten and the rareearth oxide exist on the electrode surface. This is considered to allowthe rare earth oxide to easily discharge near the interfaces, withouthaving to diffuse over a long distance.

It has been difficult to allow an oxide to be dispersed in fine formwithin tungsten. However, as is described in Patent Document 2, thetechnology of obtaining a high-density tungsten alloy via sintering at arelatively low temperature, and the industrial availability of a finerare earth oxide powder with an average particle diameter of about 1 μmor less have made it possible to obtain a tungsten alloy in which a finerare earth oxide is dispersed.

It has been found particularly effective to use oxides of Sm, Nd, Gd,and La. Moreover, it is desirable that the aforementioned rare earthoxide that is dispersed in the tungsten alloy has an average particlediameter d such that 0<d≦2.5 μm. The definition of the average particlediameter d will be described later.

Thus, in the case where oxide is dispersed in fine form, without relyingon diffusion in the interior of the alloy, excellent characteristics canbe obtained by controlling the oxide particle diameters, added amount,and the like on the electrode surface.

Hereinafter, tungsten cathodes according to embodiments of the presentinvention will be described.

Embodiment 1

A detailed observation of the discharge phenomenon from a tungstencathode should reveal that discharge is not occurring across the entireelectrode surface, but is occurring at a relatively small dischargepoint. As the current increases, the discharge point will have a largerdiameter and a higher temperature. Particularly in the case where thecurrent is 40 A or less (excluding 0 A), it generally has a diameter of20 μm or less.

In the present embodiment, the fine oxide particles in a tungstencathode are dispersed at interspaces on the order of a few μm to ten-oddμm over the electrode surface. In other words, the discharge pointincludes a small number of oxide particles, e.g., typically oneparticle.

As the dispersed oxide particles, at least one oxide selected from thegroup consisting of Sm, Nd, Gd, and La is used. It is desirable that thetotal amount of these oxides accounts for 50 vol % (volume fraction) ormore of the particles (dispersed particles) that are dispersed withinthe tungsten alloy. However, the dispersed particles may contain otherrare earth oxides or the like, such as CeO, Y₂O₃, and Tm₂O₃.

In such cases, the places where the oxide particles exist will be thefirst to become a discharge point, and therefore the ratio between thevolume and the interface (surface area) of one oxide particle isimportant. In other words, the average particle diameter d is important.As the interface increases relative to the particle volume, i.e., as theparticle diameter decreases, the electrode life becomes improved. Aswill be specifically indicated in subsequently-described Examples, inthe case where oxides of Sm, Nd, Gd, and La are used, with an averageparticle diameter d such that 0<d≦2.5 μm, an electrode life similar orsuperior to those of W—ThO₂ alloys is obtained.

However, if the added amount of dispersed particles is too small,retention of discharge may become difficult depending on the currentvalue. Therefore, in the case of allowing a current I to flow such that0<I≦40 A, given that the dispersed particles within the alloy have avolume fraction f (vol %), it is preferable that f/I≧0.083. This rangeis obtained through experimentation pertaining to thesubsequently-described Examples. Moreover, in practice, an upper limitof the added amount is defined for reasons such as coarse graining ofthe oxide and difficulties of sintering and machining.

Now, with reference to FIG. 1, a method of measuring the averageparticle diameter d of a rare earth oxide(s) and an added amount thereof(volume fraction) f will be described. Based on a 1000× opticalmicrograph taken of a polished material, they were determined throughthe following equations from the number of particles N_(S) per unit area(e.g., 10000 μm²) and the number of particles N_(L) per unit length(e.g., 100 μm) as shown in FIG. 1.

In FIG. 1, N_(S)=6/10000 μm², N_(L)=2/100 μm. At this time,average particle diameter d=4/π·N _(L) /N _(S); andparticle volume fraction f=8/3/π·N _(L) ² /N _(S).

It has been confirmed that the volume fraction f is substantially equalto a value which is calculated from an analysis value of each componentthrough chemical analysis and an oxide density. In the case where oxidesare added in combination, a volume fraction f of each oxide iscalculated based on a chemical analysis value. In the case of ananisotropic shape as in Patent Document 1, given an N_(L) in theprocessing axis direction in a vertical cross section (N_(Lv)), and that(N_(Lh)) in a direction orthogonal to the processing axis, a ratio L/Wbetween an average particle diameter L in the processing axis directionand an average particle diameter W in a direction orthogonal to theprocessing axis, an average particle diameter d, and a volume fraction fare calculated as follows. However, in the present invention, noconsideration needs to be given to L/W in practice.L/W=N _(Lv) /N _(Lh)d=4/π·(N _(Lv) /N _(Lh))^(2/3) ·N _(Lv) /N _(S)f=8/3/π·N _(Lv) ·N _(Lh) /N _(S)

EXAMPLES OF EMBODIMENT 1 AND COMPARATIVE EXAMPLES

Hereinafter, Examples of tungsten cathode materials according toEmbodiment 1 and Comparative Examples will be described.

A method of producing a tungsten cathode material will be firstdescribed. First, a tungsten powder with an average particle diameter ofabout 1 μm and rare earth oxide powders (e.g., an average particlediameter of 1 μm) as shown in Tables 3 to 6 below were mixed inpredetermined amounts. However, the start materials of the additives arenot limited to oxides; hydroxides, nitrates, carbonate, and the like,which will become oxides through pyrolysis may be used. The mixingmethod may be a wet or dry method. For components which are soluble insolvents, a so-called doping technique, which performs mixing in thestate of a solution and then dries this, is also effective. As usedherein, the average particle diameters of the aforementioned tungstenpowder and rare earth oxide powders mean a mass median diameter (mediandiameter D₅₀).

The resultant powder was CIP (Cold Isostatic Pressing compaction)-moldedinto a predetermined shape at a pressure of 400 MPa, and sintered atabout 1700° C. in a hydrogen ambient. By thus performing molding at ahigh pressure and sintering at a relatively low temperature, it becomespossible to suppress coarse graining and evaporation of the additive.

Although addition of an oxide of Tm or Lu alone was also attempted,these are not described in the tables because no sintered bodies wereobtained that were sufficiently dense in texture. In order to furtherimprove the density of the sintered bodies, an HIP treatment (HotIsostatic Pressing) may be performed after sintering.

The resultant sintered bodies were processed into predetermined shapes.A swage processing may be performed in order to obtain a rod shape.

A high-pressure mercury lamp was produced by using each resultant sampleas a cathode. The mercury lamp was lit with a current of 6 A, 10 A, or40 A, and a ratio between the illuminance at the beginning of lightingand the illuminance after 200 hours of lighting was measured. The lamptest results obtained are shown in Tables 3 to 6. Table 3 and Table 4indicate results under 6 A; Table 5 shows results under 10 A; and Table6 shows results under 40 A. The evaluations in the tables are: anythingthat exhibits similar or superior illuminance retention characteristicsreads ◯, and anything else ×, as compared to Comparative Example 1,Comparative Example 19, and Comparative Example 20, which are existingproducts having thorium added thereto.

Note that Comparative Examples 1, 19, and 20 were commercially availableW—ThO₂ alloys (THR2 manufactured by Nippon Tungsten Co., Ltd.);otherwise, Examples and Comparative Examples were produced by theaforementioned method, and tested. Each composition is indicated as vol%.

TABLE 3 illuminance f f/I after (vol d (%/ 200 hr No. additive %) (μm)A) (%) evaluation Ex. 1 Nd₂O₃ 0.5 1.5 0.083 94.0 ◯ Ex. 2 Nd₂O₃ 2 1.70.333 98.0 ◯ Ex. 3 Nd₂O₃ 4 2.2 0.667 95.0 ◯ Ex. 4 Nd₂O₃ 5 2.4 0.833 94.0◯ Ex. 5 Sm₂O₃ 0.5 2.4 0.083 94.0 ◯ Ex. 6 Sm₂O₃ 2 2.5 0.333 97.5 ◯ Ex. 7La₂O₃ 0.5 2.3 0.083 94.0 ◯ Ex. 8 La₂O₃ 2 2.4 0.333 94.0 ◯ Ex. 9 Gd₂O₃0.5 2.0 0.083 94.5 ◯ Ex. 10 Gd₂O₃ 2 2.2 0.333 97.0 ◯ Ex. 11**combination 0.6 1.5 0.100 96.0 ◯ Ex. 12 **combination 3 1.6 0.500 97.0◯ Ex. 13 **combination 4.8 1.8 0.800 95.0 ◯

Those specimens bearing the symbol on their Nos. are ComparativeExamples lying outside the scope of the present invention.

In Examples 11 to 13, 16, and 19, **combination indicates Nd₂O₃, Sm₂O₃,La₂O₃, CeO, Y₂O₃, and Tm₂O₃ being mixed in equal amounts on vol % basis.

TABLE 4 f f/I illuminance (vol d (%/ after 200 hr No. additive %) (μm)A) (%) evaluation *Com. Ex. 1 Th₂O 5 3.6 0.833 94.0 reference *Com. Ex.2 Nd₂O₃ 0.3 1.5 0.050 93.0 X *Com. Ex. 3 Nd₂O₃ 10 2.6 1.667 93.5 X *Com.Ex. 4 Sm₂O₃ 0.3 2.4 0.050 93.0 X *Com. Ex. 5 Sm₂O₃ 5 3.0 0.833 90.0 X*Com. Ex. 6 La₂O₃ 0.3 2.3 0.050 93.0 X *Com. Ex. 7 La₂O₃ 5 2.8 0.83392.5 X *Com. Ex. 8 Gd₂O₃ 0.3 2.0 0.050 93.5 X *Com. Ex. 9 Gd₂O₃ 5 3.00.833 93.0 X *Com. Ex. 10 **com- 0.3 1.4 0.050 93.5 X bination *Com. Ex.11 CeO₂ 2 2.1 0.333 93.0 X *Com. Ex. 12 CeO₂ 5 2.9 0.833 90.5 X *Com.Ex. 13 Y₂O₃ 2 2.1 0.333 93.5 X *Com. Ex. 14 Y₂O₃ 5 3.2 0.833 92.0 X*Com. Ex. 15 Pr₆O₁₁ 2 3.2 0.333 93.5 X *Com. Ex. 16 Pr₆O₁₁ 5 4.2 0.83392.5 X *Com. Ex. 17 Dy₂O₃ 2 2.0 0.333 93.5 X *Com. Ex. 18 Dy₂O₃ 5 3.20.833 90.0 X

TABLE 5 f f/I illuminance (vol d (%/ after 200 hr No. additive %) (μm)A) (%) evaluation Ex. 14 Nd₂O₃ 4 2.2 0.4 94.0 ◯ Ex. 15 Nd₂O₃ 5 2.4 0.594.0 ◯ Ex. 16 **combination 4.8 1.8 0.48 94.5 ◯ *Com. Th₂O 5 3.6 0.593.5 reference Ex. 19

TABLE 6 f f/I illuminance (vol d (%/ after 200 hr No. additive %) (μm)A) (%) evaluation Ex. 17 Nd₂O₃ 4 2.2 0.100 93.5 ◯ Ex. 18 Nd₂O₃ 5 2.40.125 93.0 ◯ Ex. 19 **combination 4.8 1.8 0.120 94.0 ◯ *Com. Th₂O 5 3.60.125 93.0 reference Ex. 20

As shown in Table 3 to Table 6, the illuminance retentioncharacteristics of Examples 1 to 19 of the present embodiment aresimilar or superior to those of the W—ThO₂ alloys of Comparative Example1, Comparative Example 19, and Comparative Example 20.

Some of the oxides (CeO, Y₂O₃, Tm₂O₃) other than Sm, Nd, Gd, and La didnot provide sufficient characteristics when added alone. However, whenthese are added in combination with oxides of Sm, Nd, Gd, and La in arange of 50 vol % or less, they become usable as indicated by Examples11, 12, 13, 16, and 19.

The necessary characteristics of mercury lamps, e.g., ignition voltage,output efficiency, output fluctuation, and the like were examined, whichindicated no particular problems. From an overall point of view, too,the tungsten cathode materials of Examples have characteristics similaror superior to those of W—ThO₂ alloys.

Thus it is indicated that, in the case where a current of 0 to 40 Aflows in a tungsten electrode, when oxides of Sm, Nd, Gd, and La with anaverage particle diameter d of 0<d≦2.5 μm are dispersed in the tungstenelectrode, satisfactory characteristics are obtained by satisfying0.083≦f/I. By allowing the aforementioned rare earth oxide particles tobe dispersed in fine form in a tungsten cathode material used as adischarge cathode material, it was ensured that sufficient interfacesbetween tungsten and the rare earth oxide existed on the cathodesurface, thus improving the discharge characteristics. Therefore, atungsten cathode material used for TIG welding for small currents,plasma spraying, plasma cutting, electro-discharge machining, dischargelamps, etc., can be improved; the use of the radioactive element thoriumcan be reduced; and a long life and a high performance can be realized.

Embodiment 2

A detailed observation of the discharge phenomenon from a tungstencathode should reveal that discharge is not occurring across theelectrode surface, but is occurring at a relatively small dischargepoint. As the current increases, the discharge point will have a largerdiameter and a higher temperature. Particularly in the case where thecurrent is as large as 10 A or more, it generally has a diameter of 10μm or more.

In a cathode material according to the present embodiment, the fineoxide particles in a tungsten alloy are dispersed at interspaces on theorder of a few μm to ten-odd μm over the electrode surface. Therefore, aplurality of oxide particles will exist at a discharge point.

As the oxide particles, at least one, or two or more oxides from amongSm, Nd, Gd, and La are used. It is desirable that the total amount ofthese oxides accounts for 50 vol % or more of the dispersed particlesthat are dispersed within the tungsten alloy. As materials other thanthe above, the dispersed particles may contain other rare earth oxides,such as CeO, Y₂O₃, and Tm₂O₃.

In such cases, the amount of interfaces between the tungsten and theoxide particles included in the discharge point (total surface area)affects the characteristics. The density of interfaces (the total areaof interfaces per unit volume) is in proportion to f/d, where f is atotal amount of the volume fraction of dispersed particles (vol %), andd is an average particle diameter (μm). In other words, the electrodelife is particularly improved when the added amount of oxide is largeand the particle diameters are fine.

As will be indicated in subsequently-described Examples, in the casewhere oxides of Sm, Nd, Gd, and La are used, when f/d≧1.67 is satisfied,an electrode life similar or superior to those of W—ThO₂ alloys isobtained. However, if the added amount is excessive, the oxide willbecome coarse grains, thus making sinter and machining difficult.Therefore, it is practically desirable that f/d≦4.

As the method of measuring an average particle diameter d and an addedamount f of oxide particles, similarly to Embodiment 1, they weredetermined through the following equations from the number of particlesN_(S) per unit area and the number of particles N_(L) per unit length asshown in FIG. 1, based on a 1000× optical micrograph taken of a polishedmaterial.

In FIG. 1, N_(S)=6/10000 μm², N_(L)=2/100 μmaverage particle diameter d=4/π·N _(L) /N _(S)particle volume fraction f=8/3/π·N _(L) ² /N _(S)

N_(L) is an amount which is in proportion to f/d as mentioned above.Moreover, it has been confirmed that f is substantially equal to a valuewhich is calculated from an analysis value of each component throughchemical analysis and an oxide density. In the case where oxides areadded in combination, a volume fraction of each oxide is calculatedbased on a chemical analysis value. In the case of an anisotropic shapeas in Patent Document 1, given an N_(L) in the processing axis directionin a vertical cross section (N_(Lv)) and that (N_(Lh)) in a directionorthogonal to the processing axis, a ratio L/W between an averageparticle diameter L in the processing axis direction and an averageparticle diameter W in a direction orthogonal to the processing axis, anaverage particle diameter d, and a volume fraction f are calculated asfollows. However, in the present invention, no consideration needs to begiven to L/W in practice.L/W=N _(Lv) /N _(Lh)d=4/π·(N _(Lv) /N _(Lh))^(2/3) ·N _(Lv) /N _(S)f=8/3/π·N _(Lv) ·N _(Lh) /N _(S)

EXAMPLES OF EMBODIMENT 2 AND COMPARATIVE EXAMPLES

Hereinafter, Examples of tungsten cathode materials according toEmbodiment 2 and Comparative Examples will be described.

First, a tungsten powder with an average particle diameter of about 1 μmand rare earth oxide powders (e.g., an average particle diameter of 1μm) as shown in Tables 7 to 10 below were mixed in predeterminedamounts. However, the start materials of additives are not limited tooxides; hydroxides, nitrates, carbonate, and the like, which will becomeoxides through pyrolysis may be used. The mixing method may be a wet ordry method. For components which are soluble in solvents, a so-calleddoping technique, which performs mixing in the state of a solution andthen dries this, is also effective.

The resultant powder was CIP molded into a predetermined shape at apressure of 400 MPa, an sintered at 1700° C. in a hydrogen ambient. Bythus performing molding at a high pressure and sintering at a relativelylow temperature, it becomes possible to suppress coarse graining andevaporation of the additive.

Although addition of an oxide of Tm or Lu alone was also attempted,these are not described as Comparative Examples because no sinteredbodies were obtained that were sufficiently dense in texture. In orderto further improve the density of the sintered bodies, an HIP treatmentmay be performed after sintering. In this case, the sintered bodydensity before the HIP treatment is 96% or more, and the degree ofcoarse graining or evaporation is small even through a high temperaturetreatment, and thus there is no practical problems.

The resultant sintered bodies were processed into predetermined shapes.A swage processing may be performed in order to obtain a rod shape.

Using a sample having been processed into a rod shape with φ02.5 mm, 100minutes of TIG welding was performed with a current of 250 A, and anamount of wear-out of the electrode was measured.

Regarding this TIG welding test, results of Examples are shown in Table7, and results of Comparative Examples are shown in Table 8. As comparedto Comparative Example 21, which is a conventional material, thosehaving superior or similar wear-out characteristics in TIG welding areevaluated as ◯, and those which are inferior are evaluated as ×.

TABLE 7 TIG amount f d f/d of wear No. additive (vol %) (μm) (%/μm) (g)evaluation Ex. 20 Nd₂O₃ 4 2.2 1.82 0.0111 ◯ Ex. 21 Nd₂O₃ 5 2.4 2.080.0085 ◯ Ex. 22 Nd₂O₃ 10 2.6 4.00 0.0052 ◯ Ex. 23 Sm₂O₃ 5 3.0 1.670.0088 ◯ Ex. 24 Sm₂O₃ 10 3.2 3.13 0.0080 ◯ Ex. 25 La₂O₃ 5 2.8 1.790.0109 ◯ Ex. 26 La₂O₃ 10 3.1 3.23 0.0093 ◯ Ex. 27 Gd₂O₃ 5 3.0 1.670.0096 ◯ Ex. 28 Gd₂O₃ 10 3.3 3.03 0.0093 ◯ Ex. 29 **combination 3 1.61.88 0.0102 ◯ Ex. 30 **combination 4.8 1.8 2.67 0.0087 ◯ Ex. 31**combination 7.2 1.9 3.79 0.0082 ◯

TABLE 8 TIG f amount (vol d f/d of wear No. additive %) (μm) (%/μm) (g)evaluation (*Com. Ex. 21) Th₂O 5 3.6 1.39 0.0120 — reference *Com. Ex.22 Nd₂O₃ 2 1.7 1.18 0.0164 X *Com. Ex. 23 Sm₂O₃ 2 2.5 0.80 0.0127 X*Com. Ex. 24 La₂O₃ 2 2.4 0.83 0.0257 X *Com. Ex. 25 Gd₂O₃ 2 2.2 0.910.0163 X *Com. Ex. 26 **com- 0.6 1.5 0.40 0.0132 X bination *Com. Ex. 27CeO₂ 2 2.1 0.95 0.0247 X *Com. Ex. 28 CeO₂ 5 2.9 1.72 0.0199 X *Com. Ex.29 Y₂O₃ 2 2.1 0.95 0.0199 X *Com. Ex. 30 Y₂O₃ 5 3.2 1.56 0.0178 X *Com.Ex. 31 Pr₆O₁₁ 2 3.2 0.63 0.0201 X *Com. Ex. 32 Pr₆O₁₁ 5 4.2 1.19 0.0175X *Com. Ex. 33 Dy₂O₃ 2 2.0 1.00 0.0265 X *Com. Ex. 34 Dy₂O₃ 5 3.2 1.560.0194 X

In Table 7, Table 8, Table 9, and Table 10, those specimens bearing thesymbol on their Nos. are Comparative Examples lying outside the scope ofthe present invention.

**Combination indicates Nd₂O₃, Sm₂O₃, La₂O₃, CeO, Y₂O₃, and Tm₂O₃ beingmixed in equal amounts on vol % basis.

As still other Examples, a high-pressure mercury lamp was produced byusing each resultant sample as a cathode, and was lit with a current of40 A or 10 A. Based on the beginning of lighting, an illuminance ratioafter 200 hours of lighting was measured. The test results obtained areshown in Table 9 and Table 10. Table 9 shows the 40 A case, and Table 10shows the 10 A case. Comparative Examples 35 and 36 are commerciallyavailable W—ThO₂ alloys, but otherwise were produced by theaforementioned method, and tested. Each composition is indicated as vol%. Those specimens having a larger added amount of additive (volumefraction f) than is indicated in Examples are omitted from the tablesbecause they had an f/d exceeding 4, only resulting in specimens whichmake the sintering step and the machining step difficult.

The results of lighting the high-pressure mercury lamps of Examples andComparative Examples with a 40 A current are shown in Table 9, and theresults of lighting them with a 10 A current are shown in Table 10. Ascompared to Comparative Examples 35 and 36, which are conventionalmaterials, those having superior or similar illuminance retentioncharacteristics are evaluated as ◯, and those which are inferior areevaluated as ×.

TABLE 9 illuminance f f/d after (vol d (%/ 200 hr No. additive %) (μm)μm) (%) evaluation Ex. 32 Nd₂O₃ 4 2.2 1.82 93.5 ◯ Ex. 33 Nd₂O₃ 5 2.42.08 93.0 ◯ Ex. 34 **combination 4.8 1.8 2.67 94.0 ◯ (*Com. Th₂O 5 3.61.39 93.0 — Ex. 35) reference

TABLE 10 f f/d illuminance (vol d (%/ after 200 hr No. additive %) (μm)μm) (%) evaluation Ex. 35 Nd₂O₃ 4 2.2 1.82 94.0 ◯ Ex. 36 Nd₂O₃ 5 2.42.08 94.0 ◯ Ex. 37 **combination 4.8 1.8 2.67 94.5 ◯ (*Com. Th₂O 5 3.61.39 93.5 — Ex. 36) reference

Examples 32 to 37 of the present embodiment are similar or inferior tothe W—ThO₂ alloys of Comparative Examples 35 and 36 in terms of theamount of wear-out and decrease in illuminance.

The oxides other than Sm, Nd, Gd, and La did not provide sufficientcharacteristics when added alone. However, when these are added incombination with oxides of Sm, Nd, Gd, and La in a range of 50 vol % orless, they become usable as indicated by Examples 29, 30, 31, 34, and37.

The necessary characteristics of mercury lamps, e.g., ignition voltage,output efficiency, output fluctuation, and the like were examined, whichindicated no particular problems. From an overall point of view, too,the tungsten cathode materials of Examples have characteristics similaror superior to those of W—ThO₂ alloys.

INDUSTRIAL APPLICABILITY

A tungsten alloy having a rare earth oxide added thereto according tothe present invention can be suitably used as a discharge material for adischarge lamp electrode, a plasma arc electrode, or a TIG weldingelectrode. It can also be used for an electro-discharge machiningelectrode or a magnetron electrode.

REFERENCE SIGNS LIST

1 oxide particle

2 unit area (10000 μm²)

3 unit length (100 μm)

4 tungsten parent phase

The invention claimed is:
 1. A tungsten cathode material in which oxideparticles are dispersed, the oxide particles containing an oxide oroxides of at least one selected from the group consisting of Sm, Nd, Gd,and La in a total amount of 50 vol % or more, wherein, an averageparticle diameter d of the oxide particles satisfies the relationship0<d≦2.5 μm; and, given a volume fraction f (vol %) of the total amountof the oxide particles in the tungsten cathode material, 0.083 f/I holdswhen a current I(A) such that 0<I≦40 A is applied to an electrode beingmade of the tungsten cathode material.
 2. A discharging methodcomprising: a step of providing a tungsten electrode in which oxideparticles are dispersed, the oxide particles containing an oxide oroxides of at least one selected from the group consisting of Sm, Nd, Gd,and La in a total amount of 50 vol % or more, the oxide particles havingan average particle diameter d in the range of 0<d≦2.5 μm; and a step ofallowing a current I such that 0<I≦40 A to flow in the tungstenelectrode, wherein, given a volume fraction f (vol %) of the totalamount of the oxide particles in the tungsten electrode, therelationship 0.083≦f/I is satisfied in the step of allowing a current toflow in the tungsten electrode.
 3. A discharging device comprising acathode and a power supply for allowing a current to flow in thecathode, wherein, the cathode is a tungsten cathode in which oxideparticles are dispersed, the oxide particles containing an oxide oroxides of at least one selected from the group consisting of Sm, Nd, Gd,and La in a total amount of 50 vol % or more, the dispersed particleshaving an average particle diameter d satisfying the relationship0<d≦2.5 μm; the power supply is arranged to allow a current of 0<I≦40 Ato flow in the cathode; and given a volume fraction f (vol %) of thetotal amount of the dispersed particles in the tungsten cathodematerial, 0.083≦f/I holds when the current I(A) applied to the cathodeis such that 0<I≦40 A.
 4. A tungsten cathode material in which oxideparticles are dispersed, the oxide particles containing an oxide oroxides of at least one selected from the group consisting of Sm, Nd, Gd,and La in a total amount of 50 vol % or more, wherein, given an averageparticle diameter d (μm) of the oxide particles and a volume fraction f(vol %) of the total amount of the oxide particles in the tungstencathode material, 1.67≦f/d≦4 holds when a current I such that 10 A≦I isapplied to an electrode being made of the tungsten cathode material. 5.A discharging method comprising: a step of providing a tungstenelectrode in which oxide particles are dispersed, the oxide particlescontaining an oxide or oxides of at least one selected from the groupconsisting of Sm, Nd, Gd, and La in a total amount of 50 vol % or more;and a step of allowing a current of 10 A or more to flow in the tungstenelectrode, wherein, given a volume fraction f (vol %) of the totalamount of the oxide particles in the tungsten electrode, an averageparticle diameter d (μm) of the oxide particles and the volume fractionf satisfy the relationship 1.67≦f/d≦4.
 6. A discharging devicecomprising a cathode and a power supply for allowing a current to flowin the cathode, wherein, the cathode is a tungsten cathode in whichoxide particles are dispersed, the oxide particles containing an oxideor oxides of at least one selected from the group consisting of Sm, Nd,Gd, and La in a total amount of 50 vol % or more; the power supply isarranged to allow a current of 10 A or more to flow in the cathode; andgiven a volume fraction f (vol %) of the total amount of the oxideparticles in the tungsten electrode, an average particle diameter d (μm)of the oxide particles and the volume fraction f satisfy therelationship 1.67≦f/d≦4.